The present invention relates to a gas-generating metal electrode for electrochemical processes, more particularly a coated titanium anode for amalgam cells consisting of spaced-apart parallel rods arranged in a horizontal plane, the surfaces of which form the working area of the electrode and have a cross section whose extent normal to said plane is greater than the extent parallel thereto.
Gas-generating metal electrodes arranged in a horizontal plane, normally the anode which is placed opposite a counter-electrode likewise arranged in a horizontal plane, normally the cathode, have to meet several criteria:
The first criterion consists in the requirement that the gas generated at the anode be carried away as rapidly as possible. To achieve this, the anode shall have a free area in the horizontal plane of arrangement, that is to say, it shall have openings or the like through which the gas being generated on the lower side of the anode can escape upward. Said free area of the anode shall be as large as possible. Experiments have shown that said free area shall be .gtoreq.approximately thirty-three percent (33%) of the projected area of the anode formed by the length times the width of the area occupied by the anode in the horizontal plane. Because the openings which can be made in perforated plates, expanded-metal grids, etc. and which form the free area through which the gas can be conducted away, are inadequate for this purpose, use is made of spaced-apart parallel rods, the upper surfaces of which form the working area of the electrode.
In rods with edged profiles such as, for example, rectangular or triangular profiles, it has been found that greater wear of the rod coating occurred on the edges. Therefore, the second criterion consists in the use of a rod profile having no sharp edges. This condition leads to the use of rods having a circular cross section.
Other conditions have to be observed in addition to the two important criteria mentioned above. One of these other conditions consists in placing the working area of the anode as close to the cathode as possible. The farther the working area of the anode is removed from the cathode, the greater the voltage drop of the cell and, thereby, the energy required for making the desired product. Since the alkali chloride electrolysis according to the amalgam process works with a mercury cathode which, due to the characteristics of the material, cannot be inherently stable and because in the event of a short circuit caused by a deformation of the cathode and resultant contact with the anode due to the high current densities in this process heavy damages arise at the anode, the operation is essentially carried out with an anode/cathode spacing of 3 mm, depending on the current density and on other conditions such as monitoring facilities, etc. Since the portion of the area of the anode running parallel to the cathode shall be minimized in order that the free area for carrying away the gas can be maximized, a portion of the working area shall be normal to the plane of the anode. Experiments have shown that the difference between the nearest and the farthest point of the working area in relation to the cathode shall be 2.0 mm maximum. Thus, in this region the coating shall primarily be deposited on the carrier, i.e., the rods and the like.
Another criterion consists in minimizing the portion of the working area of the anode which, viewed from the cathode, lies in the Stromschatten (lit. "current shadow"). Hence it follows that for the portion of the working area normal to the anode plane the radius of the round rods must be taken into consideration as another limiting value in addition to the above limiting value of 2.0 mm.
It should further be realized that the wear of the coating also depends heavily on the true current density. Therefore, it is necessary to maximize the working area of the anode for a cathode area which is predetermined by the size of the cells. The so-called projection area of the anode is determined by the length and the width of the anode. In most cases it is predetermined by the user, so that little control can be exercised. However, the projected area gives only scant information about the size of the working area of the anode and, thus, about the true current density, but an attempt should be made in any case to obtain a ratio of working area to project area of .gtoreq.1.
The above types of metal electrodes or anodes employed heretofore meets these criteria only partly.
Although in contrast to rods with edged profiles, premature wear of the coating is avoided in the already known anodes with round rods, the latter have the drawback that the portion of the working area of the anode substantially normal to the anode plane or parallel to the direction of the main flow of the electric current is relatively small, whereas the portion of the working area of the anode lying in the current shadow (viewed from the cathode) and which therefore is not, or not fully, utilized, is comparatively large. Moreover, the ratio between free and projected area of the anode is unfavorable so that only an inadequate amount of the generated gas is conducted away.
Prior art electrodes or anodes of the above type in which the rods have a rectangular cross section whose height normal to the plane of arrangement of the rods is larger than the width parallel thereto have the main drawback that the coating wears rapidly on the edges, thereby reducing the service life of these electrodes.